In various electronic devices and systems, the communication of high-speed electronic signals between components creates several challenges. Specifically, when transmitting signals at high speeds, signal deterioration, additional loss factors and attenuation losses can significantly affect the quality of signals being communicated. Consequently, it is often necessary to carefully consider the signal transmission structures being used.
As is well known, printed circuit boards (PCBs) are commonly used in today's electronic devices to support several cooperating components and provide necessary communication between these components. Similarly, complex systems often utilize multiple printed circuit boards, with appropriate interconnections allowing for the cooperation of the various circuits and/or subsystems contained on each circuit board. In one example, several circuit boards will be housed within a cabinet and connected with one another via a backplane connection system. Other connection schemes involve connectors placed directly on the circuit board, which allow a second PCB to be connected thereto.
As PCBs have evolved, the complexity of the electronic components, and the complexity of the signal transmission structures on the PCB have evolved considerably. Modern day circuit boards are typically multi-layer structures, with communication paths extending between hundreds of different components. As such, the board layout and interaction of all structures can potentially effect system operation and efficiency.
In current PCB design, there is an increased demand for high-speed communication capabilities. This often requires the communication of high-speed or high-frequency signals between two or more mounted components, with signals being carried by various communication paths extending through the circuit board structure. These communication paths may extend for relatively short distances or may extend longer distances depending upon the nature of the circuit board and the environment within which the board is used. In some cases, communication paths may simply be a few centimeters in length, while other cases require these paths to be one to two meters. Realistically, circuit board structures typically do not exceed this size, thus one to two meters is often a practical upper limit. In other systems, it is necessary to provide communication capabilities between multiple circuit boards that are space some distance apart.
As mentioned above, the high-speed signal transmission often demanded by various computing systems typically involves signals with a frequency range up to 56 gigabits per second (Gbps), or even higher. In many situations, a stripline structure is utilized to carry high-speed signals within a PCB. Transmitting signals via a stripline structure at this speed often creates several complications and creates a need to closely examine signal losses throughout the PCB.
Printed circuit boards are beneficial in many applications since they provide an inexpensive and ubiquitous way to transmit high-speed data between various electrical components in many different systems, including data communication systems. That said, circuit board structures typically display a relatively high level of electrical loss as high-speed data signals traverse the PCB. Typically, these signals are communicated through a confining transmission line structure or stripline structure which is designed to propagate the signal's electromagnetic energy. Transmission lines in general must confine this signal energy and allow it to propagate relatively unimpeded, without incurring too much electrical loss, either in the form of skin-effect metal resistive loss or as dielectric loss in the surrounding material. As this suggests, high-speed connections (i.e. high-speed communication paths) are subject to multiple detrimental effects, such as signal loss (also referred to as signal attenuation), signal deterioration and unwanted reflections, all caused by the inherent characteristics of known substrate structures.
Although the above-mentioned backplane communication structure can provide one method to support communication amongst various circuit boards and/or systems, occasionally it is necessary to provide direct connection via alternative methods. For example, it may be necessary to utilize a connector and twinax cable which can communicate precise signals between particular components within a system. In these configurations, high-speed signals are communicated using the cable assembly having connectors on each end. The board will include a connector, which facilitates the connection of the twinax cable connectors. Unfortunately, the layout of particular circuit boards requires that a signal be routed from a particular chip or component, to a connector location on the board. In this situation, the high-speed signals are routed via microstrip or stripline to an edge of the board, which is a better suited area or location for connectors and related assemblies. This approach however, incorporates all of the above-mentioned issues involving micro-strip or stripline signal communication.
As is well known, typical CPU chips or ASIC chips have high-density connection mechanisms, such as ball grid arrays (BGAs) which facilitate connection to appropriate communication pads on the circuit board. Unfortunately, these ball grid arrays, or similar high-density connection mechanisms, make it very difficult to manage signal communication, and require the well-planned use of circuit board areas. This is one reason high-speed signals are typically routed to an edge or remote location on the circuit board. While this routing does achieve the desired goal of accommodating connection to a connector socket, it does require additional space on the circuit board (or within a particular layer of the circuit board), thus creates certain additional challenges.